In electrohydraulic valve control systems that embody a valve coupled to a hydraulic actuator, it is desirable to monitor position of the actuator piston for purposes of closed-loop servo control. U.S. Pat. No. 4,749,936 discloses an electrohydraulic valve control system in which a coaxial transmission line is formed within the actuator to include a center conductor coaxial with the actuator and an outer conductor. A bead of ferrite or other suitable magnetically permeable material is magnetically coupled to the piston and surrounds the center conductor of the transmission line for altering impedance characteristics of the transmission line as a function of position of the piston with the cylinder. Position sensing electronics includes an oscillator coupled to the transmission line for launching electromagnetic radiation, and a phase detector responsive to radiation reflected from the transmission line for determining position of the piston within the actuator cylinder. In a preferred embodiment, the coaxial transmission line includes a tube, with a centrally-suspended center conductor and a slidable bead of magnetically permeable material, projecting from one end of the actuator cylinder into a central bore extending into the opposing piston. In another embodiment, the outer conductor of the transmission line is formed by the actuator cylinder, and the center conductor extends into the piston bore in sliding contact therewith as the piston moves axially of the cylinder.
U.S. Pat. No. 4,757,745 discloses an electrohydraulic valve control system that includes a variable frequency rf generator coupled through associated directional couplers to a pair of antennas that are positioned within the actuator cylinder. The antennas are physically spaced from each other in the direction of piston motion by an odd multiple of quarter-wavelengths at a nominal generator output frequency. A phase detector receives the reflected signal outputs from the directional couplers, and provides an output through an integrator to the frequency control input of the generator automatically to compensate frequency of the rf energy radiated to the cylinder, and thereby maintain electrical quarter-wavelength spacing between the antennas, against variations in dielectric properties of the hydraulic fluid due to changes in fluid temperature, etc. A second phase detector is coupled to the generator and to one antenna for generating a piston position signal. The output of the second phase detector is responsive to phase angle of energy reflected from the piston and provides a direct real-time indication of piston position to the valve control electronics.
Although the systems disclosed in the above-noted U.S. patents provide improved economy and performance as compared with previous devices for a similar purpose, improvements remain desirable. In particular, difficulties have been encountered in attempting to match the characteristic impedance of the cylinder/piston coaxial transmission line, determined by parameters and properties of the cylinder, with that of the rf generator circuitry. Specifically, it has been found that characteristic impedance of the cylinder/piston coaxial transmission line can vary widely among actuators, and does not match the standard fifty ohm characteristic impedance used throughout the microwave industry.
A general object of the present invention, therefore, is to provide apparatus for determining position of a piston within an electrohydraulic actuator that is inexpensive to implement, that is adapted to monitor motion continuously in real-time, that is accurate to a fine degree of resolution, and that is reliable over a substantial operating lifetime. Another object of the invention is to provide apparatus of a described character that automatically and closely matches the characteristic impedance of the generator circuitry to that of the piston/cylinder transmission line.
A further object of the invention is to provide a coaxial transmission system that embodies enhanced capability for matching impedance of the transmission line to impedance of the associate generator circuitry.
Yet another object of the invention is to provide a system of general utility for monitoring position of a piston within a cylinder, and having particularly application for monitoring piston position in an electrohydraulic servo valve and actuator system of the character described.